(a) Field of the Invention:
The present invention relates to a large relative aperture zoom lens system having focal length on the order of 35 mm to 70 mm for use with 35 mm single reflex cameras.
(b) Description of the Prior Art:
Out of zoom lens systems, there is known a type comprising two lens groups, i.e., a front diverging lens group and a rear converging lens group. This type of zoom lens system comprising the two lens groups are suited as a wide-angle zoom lens systems having zooming ratios on the order of 2 since it comprises a stop in the rear lens group, thereby permitting limiting within a relatively narrow range the variation of distortion, known as a defect of zoom lens systems comprising four lens groups, and shortening total length of the lens system.
For this reason, researches have recently been made on the zoom lens systems comprising two lens groups, making rapid progress and providing numerous known examples of zoom lens systems compsiting two lens groups. However, it is rather difficult to impart large relative apertures to the lens systems compsiting two lens groups and most of the conventional examples had F numbers of 3.5 at largest. For example, the zoom lens systems disclosed by Japanese Unexamined Published patent application Nos. 2548/74, 132360/78, 19022/81 and 153752/77, which are of a different lens type from the zoom lens system according to the present invention, had an F number of 3.5 and small relative apertures. Further, as a zoom lens system having a composition consisting of 7 components of 7 elements similar to that of the zoom lens system according to the present invention, there has already been known the one disclosed by Japanese Examined Published Patent Application No. 13334/80. However, this zoom lens system has low performance.
The impossibility to obtain a zoom lens system having a large relative aperture is attributed to the basic composition of the zoom lens systems that they comprise a front diverging lens group. Speaking concretely, the rays strongly diverged by the front lens group remarkably enlarge relative aperture of the rear lens group at the stage of incidence on the latter. Accordingly, under-corrected spherical aberration is produced especially at the tele position of the zoom lens systems. When this spherical aberration is corrected by a triplet type of rear lens group in the manner adopted in the conventional zoom lens systems, the image side surface of the negative lens component arranged in the triplet type of lens group has too strong power, thereby curving astigmatism characteristic at the wide position and degrading image plane characteristic. In addition, it aggravates flare due to coma. These disadvantages degrade performance of the zoom lens systems and image contrast.